The present invention relates to a two-cycle engine with a direct fuel injection system, and more particularly to a combustion chamber of the engine for promoting a stratified charge.
In a two-cycle engine, a cycle is completed with one rotation of the crankshaft so that air cannot be induced by itself. As the simplest method of producing scavenge air, a crankcase scavenging system is used.
In an ordinary crankcase scavenging two-stroke engine, vacuum is produced in an enclosed crankcase of the engine to pump fresh air into the crankcase when a piston ascends. When the piston descends, burned gas is discharged through an exhaust port while the fresh air in the crankcase is induced into a cylinder, thereby scavenging the cylinder. However, since the intake air quantity is small at a light engine load, the scavenging becomes insufficient, resulting in misfiring and hence in an irregular engine operation. In addition, a torque characteristic corresponding to the engine operating conditions becomes unstable. The oil consumption increases and a part of the fuel escapes from the cylinder, thereby resulting in an increase in the fuel consumption and toxic exhaust gas. Moreover, vacuum in the crankcase is insufficient for scavenging at a heavy engine load.
In order to resolve such problems, a two-stroke engine with a separate scavenge pump is used. The scavenge pump is driven to generate a scavenge pressure thereby delivering a sufficient amount of the fresh air into the cylinder. The engine is further provided with a direct fuel injection system so as to prevent the fuel from escaping through the exhaust port.
However, particularly in a low engine load range, a large amount of the burned gas remains in the combustion chamber of the engine to dilute the fuel and the scavenge air therein so that the combustible mixture is not perfectly burned. As a rich mixture of the fuel and the fresh air is directed toward a spark plug, a stratified charge increases combustion efficiency.
Japanese Patent Application Laid-Open 62-32214 discloses such a two-cycle engine. The two-cycle engine has a hemispheric cavity formed at a top of the combustion chamber, so that the cavity promotes turbelence of the fresh air in the vertical direction. The injected fuel is mixed with the turbulent air, and the mixture comes near the spark plug.
However, since a piston crown of the engine is flat, turbulent air flow is splashed by the piston crown so that the turbulence is attenuated. In addition, a part of the fuel injected from the injector is mixed with the remaining burned gas instead of the fresh air. Therefore, high combustion efficiency can not be expected.